1. Field of Invention
This invention relates generally to power electronic inverter systems, and more particularly to methods and apparatus to reduce noise, vibration and harshness (NVH) associated with an electric machine for a vehicle.
2. Background Art
Electric and hybrid electric vehicles can employ electrical energy for propulsion via an electric drive system that can include a power circuit, such as a power electronics inverter, coupled to an electric machine. In this arrangement, the power circuit can control the transfer of power between a power source and the electric machine to drive a load, such as the vehicle transaxle. For a three-phase, AC electric machine, the power circuit can include an inverter with three phase legs, each leg comprising switches that can be individually controlled to provide a desired inverter output. As an example, drive signals that are a function of an electric machine's torque requirement can be provided to the inverter by an inverter control.
Pulse width modulation (PWM) signals are often used as drive signals for an inverter. The desired fundamental voltage commands can be modulated by a carrier signal to produce a series of pulses that turn inverter switches on and off in order to produce the desired voltages on the coils of a multi-phase electric machine. Employing lower switching frequencies can reduce switching losses in the inverter. However, switching at frequencies within the audible spectrum can produce unpleasant high pitched whining and whistling noises that irritate automobile operators and passengers.
Efforts to reduce audible electric machine noise often rely on the use of carrier frequencies higher than the audible spectrum of the typical human ear. For example, U.S. Patent Application Publication No. US20090115362, filed by Subrata Saha et al., assigned to Aisin AW Company Limited, and published May 7, 2009, discloses a strategy to suppress side band noise by switching from a low frequency carrier (5 kHz) mode to a high frequency carrier (7.5 kHz) mode under predetermined target torque and rotational speed conditions, and switching back to the low frequency mode when other predetermined target torque and rotational speed conditions are present. While employing a carrier frequency of 7.5 kHz can move the switching frequency to the upper end of the audible range where the human ear is less sensitive, the higher switching frequency can result in higher switching losses, which can decrease fuel efficiency for an electric or hybrid electric vehicle.
U.S. Patent Publication US20100052583, filed by Naoyoshi Takaatsu et al., assigned to Toyota Jidosita, and published Mar. 4, 2010, teaches changing PWM strategy and frequency in an attempt to reduce noise and improve fuel efficiency. Takaatsu discloses a vehicle that includes a motor for driving wheels, an inverter to drive the motor, and a control device to perform PWM control of the inverter. The control device performs synchronous PWM control in a case where an electric current supplied to the motor by the inverter or torque generated in the motor is larger than a threshold value; and performs synchronous PWM control or non-synchronous PWM control in a case where the electric current or the torque is smaller than the threshold value, and sets carrier frequency or a pulse number of the PWM control to be higher in the case where the electric current or the torque is larger than the threshold value. Both synchronous and non-synchronous control circuits are required, with switching performed between the two of them. While asynchronous control can use an arbitrary frequency, synchronous control requires integer multiples of a base carrier frequency. While the Takaatsu method and system may be effective to reduce noise in some instances, it requires additional control circuitry and requires different PWM strategies for different operating conditions. For example, in some motor operating regions a strategy is implemented for noise reduction, while in a different operating region a strategy is implemented to reduce inverter losses. Both disclosures mentioned above require real-time processing to assess the current electric machine state, and designation of a PWM carrier frequency that is dependent on current and torque requirements. While perhaps adequate for their intended purposes, neither disclosure teaches a method for providing PWM control signals that can reduce electric machine noise in an optimized manner that is independent of current and torque conditions. Other solutions, such as randomization of incremental frequencies around 7.5 kHz, such as 7.75 kHz and 7.25 kHz, help mitigate switching noise, but increase switching losses and decrease fuel economy.